Electronic systems have become a ubiquitous fixture in modern society. These electronic systems range from simple, hand-held calculators to more complex systems including computers, personal digital assistants (PDAs), embedded controllers and complex satellite imaging and communications systems. Many electronic systems include a microprocessor that performs one or more functions based on data provided to the microprocessor. This data is typically stored in a memory device of the electronic system such as a common dynamic random access memory (DRAM) device.
A DRAM typically includes an array of memory cells that store data as binary values, e.g., 1's and 0's. In a conventional DRAM, the data is stored by controlling the charge on capacitors in each cell of the DRAM. Data in the array is “randomly accessible” since a processor can retrieve data from any location in memory by providing the appropriate address to the memory device.
One problem with conventional DRAM is that the device is “volatile.” This means that when power is turned off to the system using the DRAM, the data in the memory device is lost.
Non-volatile memory devices exist and are also in wide use today. One type of non-volatile memory is referred to as Flash memory. Flash memory is commonly used in many applications like cell phones, PDAs, and the like. Conventional Flash memory store data on “floating gates.” When power is removed from the electronic system, the floating gates retain their current charge so that data is not lost when power is removed.
Conventional Flash technology is not without problems. One problem with Flash memory is the speed of operation. Flash memory is much slower than conventional DRAM. Conventional DRAM cells can write data in a fews tens of nanoseconds whereas Flash cells can take at least a microsecond to write the same data. Thus, Flash cells are hundreds of times slower than comparable DRAM cells. When millions of bits are being stored, this timing can produce significant delays. Further, Flash memory cells begin to break down much more quickly than DRAM cells.
Researchers have been working on developing a new non-volatile memory referred to as magneto-resistive random access memory (MRAM). Unlike conventional DRAM, which uses electrical cells (e.g., capacitors) to store data, MRAM uses magnetic cells. Because magnetic memory cells maintain their state even when power is removed, MRAM possesses a distinct advantage over electrical cells.
In one form of MRAM technology, two small magnetic layers separated by a thin insulating layer typically make up each memory cell, forming a tiny magnetic “sandwich.” Each magnetic layer behaves like a tiny bar magnet, with a north pole and south pole, called a magnetic “moment.” The moments of the two magnetic layers can be aligned either parallel (north poles pointing in the same direction) or antiparallel (north poles pointing in opposite directions) to each other. These two states correspond to the binary states—the 1's and 0's—of the memory. The memory writing process aligns the magnetic moments, while the memory reading process detects the alignment.
In MRAM technology, data is read from a memory cell by determining the orientation of the magnetic moments in the two layers of magnetic material in the cell. Passing a small electric current directly through the memory cell accomplishes this: when the moments are parallel, the resistance of the memory cell is smaller than when the moments are not parallel. Even though there is an insulating layer between the magnetic layers, the insulating layer is so thin that electrons can “tunnel” through the insulating layer from one magnetic layer to the other.
To write to an MRAM cell, currents pass through wires close to (but not connected to) the magnetic cell. Because any current through a wire generates a magnetic field, this field can change the direction of the magnetic moment of the magnetic material in the magnetic cell. The arrangement of the wires and cells is called a cross-point architecture: the magnetic junctions are set up along the intersection points of a grid. Word lines run in parallel on one side of the magnetic cells. Bit lines runs on a side of the magnetic cells opposite the word lines. The bit lines are perpendicular to the set of word lines below. Like coordinates on a map, choosing one particular word line and one particular bit line uniquely specifies one of the memory cells. To write to a particular cell (bit), a current is passed through the word line and bit line that intersect at that particular cell. Only the cell at the crosspoint of the word line and the bit line sees the magnetic fields from both currents and changes state.
One difficulty with reading data from an MRAM cell is a small difference in resistance exists between the two logic states of the cell. In some cases, this small difference in resistance makes it difficult to reliably read data from the cell. Thus, there is a need in the art for an improved technique for reading data from an MRAM cell.